1. Field of Invention
This invention is directed to the fabrication of structures with high aspect ratio features. In particular, this invention is directed to the fabrication of microchannel collimating structures.
2. Description of Related Art
Features which have high aspect ratios, e.g., voids which are narrow but relatively deep, or columns which are narrow but relatively high, are fabricated using various thin film manufacturing techniques. For example, vias in electronic integrated circuitry have required development of processes which are capable of repeatably controlling the feature dimensions. Vias are required to be small, for example, in order to increase the packing density of electronic structures built in a die. However, they are also required to be quite deep, as the increasing complexity of integrated circuits often requires electrical access to layers buried far below the surface.
Another structure requiring high aspect ratio features is a microchannel collimating array. A microchannel light collimating array is a sheet of material with an array of parallel tubes that transmit light through the material but absorb light traveling at angles from the axis (measured in radians) greater than the reciprocal of the aspect ratio, where the aspect ratio is given by the height divided by the diameter. Light rays traveling along or nearly along the axis of the tube are transmitted through the tube, whereas off-axis light rays are captured by the absorbing tube walls. Therefore, the light transmitted through the microchannel array tends to be collimated in a direction parallel to the tube axes.
Because the tubes only transmit light traveling nearly along the axis of the tubes, microchannel collimating arrays can be used to spatially resolve small areas on a surface, and distinguish the light coming from these small areas, from light coming from adjacent areas. Therefore, microchannel collimating arrays can improve the resolution of large area detectors by allowing light from only a small region on the surface to reach the detector. Such microchannel collimating arrays therefore allow the large area detectors to operate in the near field, without using optical lenses, to collect light from a small area of the surface.
Microchannel collimating arrays can also be used to collimate light of frequencies for which ordinary lenses are not available. For example, optical lens materials may not be sufficiently transmissive in, for example, the ultra-violet region of the spectrum.
In the ultra-violet region, microchannel collimating arrays can be used as a barrier between regions at different pressures within an apparatus, as well as a collimator. In this situation, a transmissive barrier is required to separate the different pressure regions. However, as mentioned above, transmissive materials are not available in some portions of the spectrum. Microchannel collimating arrays can provide such a barrier, because the dimensions of the transmissive tube are sufficiently small to impede the flow of gas through the tube, thereby allowing a pressure differential to be maintained across the microchannel collimating array. For example, by disposing a high vacuum turbopump on one side of the microchannel collimating array, and a mechanical roughing pump on the other side, a pressure differential of several hundreds of torr can be maintained.